Different prior art procedures for detecting and controlling the formation of frost or ice on a heat pump outdoor heat exchange coil have been performed with varying degrees of success. These procedures include cyclical deicing, sensing air pressure drop across the outdoor coil, sensing temperature differences between the air and the outdoor coil, photo-optical responses from the frost (reflectivity), capacitance change due to the frost build up as well as tactile change due to ice formation on the coil. While some of these methods directly sense the formation of frost or ice, others use secondary effects, such as air pressure drop or thermodynamic and heat transfer changes in the system for initiation and/or termination of a deicing cycle.
One prior art proposal for defrosting makes use of a power factor change of an outdoor fan motor as ice builds up on the outdoor coil. The ice impedes air flow and changes the loading on the fan motor. This system is dependent on motor selection for the fan.
Photo-optical systems have been used which are positioned to view heat exchange fins or tubes on outdoor heat exchange coils and detect the presence of ice by observing changes in reflectivity of a light source. The ability to detect hoar frost and/or glare ice and differentiate the thickness of the ice build-up have been problems for these systems.
Measuring the capacitance of the frost has been tried with minimal success, due to the variability of ice, sensitivity of the signal, and critical placement of metal plates between which the frost build up occurs.
Fluidic sensors use "Coanda principles", in which air is passed thru one leg of a flow path and diverted to a second leg when a blockage signal is received. These sensors experience problems associated with dust and dirt clogging the filters protecting the small passages used in the fluidic sensor.
Still other methods employ tactile means of detecting the presence of ice, or employ the freezing effects of ice to increase friction and loading on a movable lever mechanism. These systems can only be employed on certain coil designs and adjustability has been a problem.
Other systems use electromechanically-operated timing devices to start a defrost cycle. They either reverse the refrigerant flow through the outdoor coil, turn on heaters, or blow hot gas over the coil.
These timing systems are simple and reliable. They do not, however, defrost "on demand" and therefore utilize energy for defrosting when there may not be a need to deice. Since it has been shown that a light hoar frost may even improve the effectiveness of some heat transfer surfaces the timed defrost systems appear to be undesirable.
Use of temperature responsive devices in combination with a clock-operated timer makes the defrosting "permissive". One example of this type of process is to initiate a defrost cycle only when outdoor temperatures fall below 32.degree. F.
Electromechanical timing devices can generally also be programmed for both frequency and duration of the deice cycle. A degree of selectability is desirable to accommodate both variations in climate and idiosyncracies of individual heat pumps.
Integration of temperature responsive elements with a clock driven mechanism offers both cost effectiveness and ease of installation and servicing of the devices. These systems, when properly programmed, will perform reasonably well under most climatic conditions and offer energy savings over the inflexible cyclical defrost procedures.
Defrost systems capable of sensing two temperatures (the outdoor ambient and the outdoor coil temperature) can provide a signal when the insulating effect of frost on the coil causes the air and outdoor coil surface temperature difference to increase to a predetermined value. Such systems provide reasonable performance when properly installed and adjusted. They provide a form of "demand" defrost which is more energy conserving than cyclic heat pump defrost controls.
The effectiveness of defrost systems using the temperature difference between outdoor air and the outdoor heat exchange coil is decreased at low temperatures. At low temperatures the heat transfer capacity of the heat pump is decreased and a fully frosted heat exchange coil doesn't deviate as greatly from outdoor air temperature. To activate defrosting at low temperatures, the threshold temperature difference between coil and air temperature must be smaller. Furthermore, the temperature difference between an unfrosted coil and a fully frosted coil is reduced markedly from differentials encountered at higher outdoor air temperatures. This can lead to false defrosting if the coil temperature fluctuates for reasons other than a frosted coil.
Many heat pump expansion valves meter refrigerant to the outdoor coil depending on the heating demands sensed inside the building. These valves commonly include an expansion valve member driven between fully opened and closed positions by an electric motor and drive train which, in turn, are operated in response to sensed conditions. When the expansion valve first opens the valve member can oscillate as the valve drive and condition sensing devices seek a stable, appropriate setting. This "hunting" behaviour of the valve member causes the outdoor heat exchange coil temperature to oscillate. If the oscillatory variations in coil temperature are large enough, the difference between sensed outdoor air temperature and sensed coil temperature become sufficiently great to indicate a defrost is necessary. This is caused by a temporarily unstable expansion valve and not by a frosted outdoor coil.
Expansion valve instability can cause the coil temperature to oscillate by more than 5 degrees Fahrenheit. One solution to this temporary instability problem has been to increase the temperature differential threshold level required to begin defrosting the coil so that these fluctuations will not initiate a defrost. This solution has made the systems particularly insensitive to needs to defrost at low outdoor temperatures and, in addition, when the system refrigerant charge becomes low the system will not be defrosted.